Mobile wireless communications systems, such as GSM/GPRS (Global System Mobile/General Packet Radio Service) systems, include base stations that can communicate with large numbers of mobile stations. The base stations periodically transmit a reference signal known as a frequency burst (FB) on a broadcast control channel (BCCH) to enable the mobile stations to synchronize with the base stations. The purposes of the frequency burst are twofold, namely frequency synchronization and coarse timing. First, with respect to frequency synchronization of the mobile station, the frequency burst enables correction of the frequency offset or error between the local carrier frequency of the mobile station and the carrier frequency of the base station. Second, with respect to coarse timing, approximate time alignment with the beginning of the frequency burst is performed so that a fine tuning can be conducted in the later acquisition of a sync burst (SB) from the base station.
As the radio frequency bands employed for wireless communication become more crowded, the possibility of interference from other radio frequency sources has increased substantially. For example, it is possible that a GSM wireless network may need to co-exist with legacy analog cellular networks and other digital cellular networks in the same frequency band. For this reason, a frequency burst acquisition technique should be able to distinguish between a frequency burst and a continuous wave (CW) signal or a very narrowband signal of other potential interfering systems.
A key factor in frequency burst acquisition by the mobile station is its ability to tolerate a wide range of frequency offset. Frequency offset in the mobile station is typically caused by temperature variation of the crystal that is used as a time base reference for the mobile station. Even when crystal accuracy is enhanced in a digital compensated crystal oscillator (DCXO), the undesired offset may still be substantial due to factors such as crystal aging, factory calibration inaccuracy and temperature variation. Expensive crystals tend to be more accurate but less desirable from the price perspective. Inexpensive crystals are more susceptible to offset problems but are more desirable from the price perspective.
What is needed is an apparatus and methodology which achieves accurate frequency burst acquisition with large frequency offset tolerance. Moreover, it is desirable that this be achieved while using relatively inexpensive crystals or other references as time bases.